1. Field of the Invention
This invention relates to a prosthetic implant, and more particularly, with a prosthetic implant having a biplanar angulation to permit insertion straight along a diagonal angle into a disk space.
2. Description of the Related Art
Spinal fusion is a commonly performed procedure. In a typical spinal fusion operation, a surgeon places a mechanical container, commonly known as a cage, between at least two adjacent vertebrae of the spine. This container contains or is later filled with bone graft which eventually incorporates into the adjacent vertebrae and creates a solid fusion. Interbody cages are placed in the disk space following removal of the disk. The cage can be surgically placed via several approaches, such as anteriorly through the abdomen, posteriorly through the spinal canal, posterolaterally through the neuroforamen of the vertebra, and transversely from a side of the spine. A goal of the surgical approach is to minimize trauma to adjacent structures and incision size.
A challenge to the placement of cages is the attainment of proper fitment between the adjacent vertebrae. It is important that a cage surface fits flushly against the endplates of the adjacent vertebrae. If a cage fits poorly, the cage could loosen, causing poor fixation and potential re-operation. The disk space, where the cage is placed, is not parallel. The space is angulated such that it is wider anteriorly than posteriorly. This angulation is termed lordosis.
Several cage designs have been proposed in the prior art. Brantigan (U.S. Pat. No. 4,834,757) describes a square shaped cage which is impacted in between vertebrae. Michelson (U.S. Pat. No. 5,015,247) describes a straight threaded cage which is screwed into the disk space. Brantigan (U.S. Pat. No. 5,425,772) and Michelson (U.S. Pat. No. 6,302,914) describe a cage with a built in single-plane lordotic angle to improve fitment between adjacent vertebrae. These cages are designed for posterior or anterior placement.
The placement of cages from posterior, lateral, and anterior approaches raise concerns about potential impingement upon important anatomical structures. Such structures are the spinal canal, the spinal nerves, and the abdominal vasculature, respectively. The posterolateral approach, also called the transforamenal approach, is gaining popularity as the preferred approach for the placement of intervertebral fusion cages. Typically, cages designed for this approach are “banana shaped,” as exemplified by Varga, et al (U.S. Pat. No. 6,579,318).
A significant difficulty with banana-type cages is that their placement requires cage rotation at the time of placement to seat the cage in place. The cage is inserted at a 45 degree lateral angle into the disk space (the maximum angle limited by anatomical structures) and is then rotated a further 45 degrees within the disk space for proper placement. This rotational step is difficult in that it occurs blindly inside the disk space. Incomplete rotation frequently occurs, resulting in poor cage fitment, with potential loosening and reoperation.
If a Brantigan (U.S. Pat. No. 4,834,757) cage were inserted in a diagonal or non-straight trajectory, it would not have flush contact with the adjacent vertebrae due to the lordotic angle of the disk space. Furthermore, a standard lordotic cage, such as Michelson (U.S. Pat. No. 6,302,914), would have its lordotic angle in the incorrect orientation for proper fitment if placed diagonally.
What is needed is a cage that can be placed via the posterolateral approach in a straight manner and that does not require a further rotation for placement. Desirably, such a cage will be placed at a straight angle relative to an anterior-posterior axis and, preferably, at a diagonal or angle, such as approximately 45 degrees and reside in a diagonal configuration within the disk space.